The present invention relates generally to a novel and most efficient scrubber for the treatment of flue gases from power generation plants or from chemical plants, in order to absorb objectionable impurities, such as SO2, SO3, HCl, nitrous oxides, into, a water-based solution, in which solids are not substantially precipitated.
Flue gases from power generation plants and boiler houses contain variable amounts of SO2, SO3, nitrous oxides or acids and similar impurities of acidic nature, deriving from the fuel burned Similarly, exit gases from many chemical plants contain such compounds or similar impurities. These impurities are known to be hazardous to the environment and it is generally required by statutory regulations to treat these gases before their discharge to the atmosphere, in order to reduce, as far as possible, the content of these objectionable impurities. In some cases, CO2 is also included in this category.
There are many processes and methods actually used, generally grouped under the name of wet Flue Gases Desulfurization (FGD), all of which consisting of contacting the flue gases with an aqueous solution or slurry, having a basic reaction, in order to absorb the objectionable impurities. These processes and methods could be subdivided into:
a) alkaline slurry containing limestone or burned lime, generally resulting in a slurry of calcium sulfite and/or sulfate crystals and a contaminated aqueous solution bleed
b) alkaline solution of hydroxides of either alkali (potassium, sodium), alkaline earth (calcium, magnesium) or ammonium, resulting in solutions of sulfite and/or sulfate salts in which solids are not substantially precipitated, and
c) dilute saline solutions, such as sea water, used on a once-through basis.
An intimate contact between the flue gases and the slurry or solution is required in order to absorb the acidic impurities into the aqueous solutions and react them with the basic-reacting component. Such contact is carried-out in specially-designed equipment, termed contactor, absorber or scrubber. The term scrubber will be used in this application, although the other terms are included in the scope of the present invention.
Generally, the industrial scrubbers used consist of vertical columns, in which the slurry or solution is flowing downwards and the gases may flow either downwards (that is co-current contact) or upwards (that is counter-current contact). In smaller operations in the past, packed columns and/or tray columns were used but counter-current spray-columns are now used more extensively. Although the packed or tray columns give a more efficient contact than spray columns, they cannot operate with concentrated slurry, are much more expensive to install and create a larger pressure drop on the gas flow.
The design of industrial scrubbers must accommodate contradictory requirements; on one hand to adjust the liquid/gas volumetric flow-rates to a workable range, by a significant internal liquid recycle, and on the other hand to maintain a very small forwards net liquid flow, needed to provide a concentrated solution for further processing.
A counter-current spray column generally consists (see illustration in FIG. 1) in an empty vertical cylindrical chamber with the following process steps:
a) A solution or slurry is pumped under pressure and sprayed in the upper part, by means of a large number of spray nozzles, organized in a manifold system covering the whole horizontal cross section of the column, at one or several different heights.
b) The resulting drops flow downwards and are collected into a sump at the lower end of the column; a large part of the drops impinge with one another in their flight and coalesces into larger drops.
c) The gases are introduced from a horizontal duct at one side of the column, and their flow-lines must perform a 90xc2x0 C. turn, before they can flow vertically upwards, against the flow of drops; as a result, uneven velocities and dead-zones are created.
d) The vertical contact results in both partial absorption and temperature equilibration, as the gases can be cooled by water evaporation; sometimes small drops may be entrained upwards by the upraising gas flow.
e) Above the upper row of spray nozzles, the gases are usually demisted from entrained droplets before being discharged.
f) The largest part of the solution or slurry from the sump is pumped back to the sprayed nozzles and fresh solution or slurry is added to this stream; as a result, a bleed stream of solution or slurry is removed continuously from the sump and sent to a further processing step, which is an integral and necessary part of any FGD process.
This generally accepted configuration of spray-column scrubber has been used in a large number of FGD plants producing calcium sulfite or sulfate slurries, since it is simple to conceive and construct, and can be operated with more or less concentrated slurries. It has however a number of inherent deficiencies which impair on its effective applications in other processes typical examples are:
a) The scrubber operates as one single equilibrium stage, at most, due to the facts that the large liquid recirculation rate is from end-to-start and that the concentration changes in the liquid cycle are relatively small. Any process requiring more than one equilibrium stage (in order to obtain a lower residual concentration of objectionable impurities in the exit gases) cannot be performed in a single spray column scrubber. The term counter-current is misleading here, since it refers only to the hydrodynamic flows but not to the process results. In addition, in order to approach a one-stage equilibrium and to provide for the necessary mass transfer driving force, the circulation load must be increased considerably and excess reactants must be maintained, leading to waste of unused reactant in the bleed stream.
b) Relatively low gas flow vertical velocities, generally less than 1 to 3 meters/second, are used to limit back-mixing and entrainment of liquid/slurry drops, which would be counterproductive to the absorption process. The lower gas velocities, in connection with the large volumetric flows of flue gases, generally result in industrial columns with very large diameters.
c) These large diameters are combined with the significant heights required for the different duties, (i.e. collecting sump, gas turning section, contacting section, separation and demisting) result into very bulky columns with costly construction and foundations problems.
d) These significant heights also increase the pumping energy consumed for slurry recirculation, using more expensive high pressure pumps.
The object of the present invention is generally to provide a novel and more efficient scrubber, which avoids, or at least reduces significantly, the above mentioned inherent deficiencies of the spray column scrubber and is particularly suited for FGD processes aiming at high elimination efficiency and using a water-based solution in which solids are not substantially precipitated.
Within this class of FGD processes, those using ammonia as the basic reactant are of special interest, since these produce a concentrated solution of ammonium sulfate which can be processed profitably into a variety of fertilizers.
The novel scrubber comprises (see illustration in FIG. 2) a horizontal chamber with a rectangular cross section, divided along the horizontal axis into a number of compartments by demisters. The gases enter through a duct at one end and exit through a duct at the other end. The xe2x80x9cpicket-fencexe2x80x9d demisters between the compartments assure that gas flow lines are spread evenly over the whole cross section area and that entrainment of drops between compartments is kept to a minimum. If needed for layout constraints or convenience, the horizontal axis can be bent or turned in any direction, or even arranged as a horse-shoe complete turn-around.
In each compartment, there is a liquid-collecting sump and a centrifugal pump (except possibly for the last one) which distribute the liquid to a series of 3-5 xe2x80x9cfailing curtainsxe2x80x9d of spray nozzles, perpendicularly to the gas flow. The liquid drops return to the sump. The liquid sumps are arranged so that excess liquid from one sump overflows into the previous one, without possibility of liquid back-flow. Water or a process solution is introduced in the last compartment. Solutions of increasing concentrations are obtained in the overflows from the different compartments, and a concentrated solution is obtained from the first compartment""s sump, which overflows into a collecting tank. The base reactant can be introduced in any one, or several compartments, as more convenient for the process considered. It could also be sprayed into the hot flue gases entering the scrubber.
The gas flow is contacted thoroughly in the series of compartments with liquids with different controlled concentrations establishing a true multi-stages, i.e., a number of compartments, counter-current process configuration where directions for gas flow and liquid flow are different as described above. In last compartment may serve also for final demisting before the gases exit.
The novel scrubber avoids, or at least reduces significantly the inherent deficiencies of the spray column scrubber listed above, in view of the following:
a) It allows a true multi-stages counter-current process configuration, instead of a single stage, by separating between the internal recycle needed to adjust the liquid/gas volumetric flow-rates to a workable range, and the very small forwards net liquid flow needed to provide a concentrated solution for further processing. This results into
a smaller contact volume is required, due to the higher mass transfer driving forces, and/or
a more concentrated bleed solution is obtained for further processing, and
a lower residual concentration of objectionable impurities remains in the outgoing gases
b) It allows working with much higher gas velocities than in a spray column, since the gas is flowing perpendicularly to the drops gravity force, so that the drops trajectory can only be moved sideways, until they impinge with the vertical xe2x80x9cpicket-fencexe2x80x9d demisters. This results into much smaller cross section areas and xe2x80x9cradialxe2x80x9d dimensions.
c) The horizontal configuration allows for light-weight construction, at any convenient height, and easily adaptable to industrial layouts, as it can be bent and installed on the roof or at roof level. The sumps needed are very shallow and could be slopped in any direction, as convenient.
d) The pumping head needed is lower, by at least a factor of five, thus the energy cost is much reduced and the pumps need not be high pressure models.
e) The separation of the solutions in the different compartments makes possible the use of feed-back control on the input of reactants for efficient adjustment to fluctuations in operating conditions, and consequently, minimizing the usage and waste of such reactants.